docs: def write_fastq(filename):
      """
      return a handle for FASTQ writing, handling gzipped files
      """
      if filename:
          if filename.endswith('gz'):
              filename_fh = gzip.open(filename, mode='wb')
          else:
              filename_fh = open(filename, mode='w')
      else:
          filename_fh = None
      return filename_fh